The invention presented here is in the domain of devices for the measurement of the flow rate of fluids in a channel.
The mass flow sensor is designed to be incorporated into a mass flow meter placed in a system for the management and control of the circulation of high purity gas, for example.
These mass flow meters usually consist of a capillary tube for the circulation of fluid, on which the representative measurements of the flow are carried out, and which is arranged in parallel with the main circuit of fluid circulation.
Numerous types of mass flow meters are already known to the professional. The principle of the flow sensors at the Wheatstone bridge and the measurement of the temperature difference has been known since 1947. They are most often based on a local heating of the passing fluid in the capillary tube, and a measurement of the variation in the resistance of the resistive components as a function of the temperature. The aforementioned measurement is representative of the flow of the gas in the tube, and thus of the flow rate. The two measurement resistors, of equal value, possess a large resistance and a large coefficient of variation with the temperature, are most often simply wound around the insulated capillary tube. These resistors, supplied with a constant current, heat up the tube but thus function also as the mechanisms for measuring the temperature. One of the disadvantages of this type of sensor is the sizable thermal mass of the wound resistors, which generates an equally long response time.
Among other known devices, the European patent application 0 131 318 B1 (Bronkhorst 1984) could be cited, which poses the problem of deriving the zero point of measurement when the fluid itself has a variable temperature, and suggests a mounting principle for the flow sensor based on four resistors sensitive to temperature deposited in a manner, not detailed, on a layer, the nature of which is not specified. The resistors are mounted in the Wheatstone bridge in a customary manner.
In another application, (EP 0 395 126 B1), Bronkhorst proposes a geometry of the tube having a very elongated U, and equipped with a series of thermocouples placed symmetrically and a central heating resistor in two parts, possibly with Peltier cooling components, for handling problems of errors in measurement associated with a circulation of air to the outside of the sensor or from internal convection to the capillary tube.
The U.S. Pat. No. 5, 373 737 (Goldstar 1993) presents a mass flow sensor which is insensitive to variations in the external temperature, using two resistors wound to the outside of the fluid circulation tube, mounted in a Wheatstone bridge, and a cooling element placed in a thermally insulated enclosed space.
Similarly, U.S. Pat. No. 5,410,912 reveals a mass flow sensor in which two resistors are used with a mounting in a bridge. The aforementioned sensor is independent from the variations in the ambient temperature due to the intermediate action of an enclosed space specific to the tube at the point of measurement. One of these resistors is used for heating and the other for measurement of the resistance as a function of the temperature reached.
There are several other patent documents involving mass flow sensors. The article xe2x80x9cA calibration system for calorimetric mass flow devicesxe2x80x9d (Widmer, Felhmann, Rehwald, J. Phys., E: Sci. Instr Vol 15 1982 pp 213) presents the global manner of diverse technologies existing for flow meters since 1900 and the changes. One article for the presentation of the background of the recent technical evolution of mass flow meters is provided by xe2x80x9cRecent Advances in Mass Flow Controlsxe2x80x9d, pp 86, Solid State Technology, 9/94.
As it is observed, several designs for thermal fluid flow sensors have been proposed to solve problems of measurement precision if the sensor is outside the capillary tube, of corrosion if the sensor is placed inside the tube, of deriving the reference point as a function of the intrinsic temperature of the fluid, of the interference phenomenon of transfer of heat to the inside or outside of the tube, of imprecision in manufacturing or of non-reproducibility in manufacturing.
Sensors of a low-flow (10 to 20 cc/minute) have many principle faults: response time, less extensive range of measurement and precision.
The response time of the existing devices is on the order of tens of seconds at the level of the sensor (time measurement of convergence of oscillations to 67% of their equilibrium value), this time being improved (typically reduced by a factor of three) by the use of the PID electronic card (Proportional Integrator Derivation) which anticipates measurement oscillations of the sensor and predicts the value of convergence. A digital card giving values specified from modeling parameters of oscillations of the sensors allows further improvement of the time, in an artificial manner.
The problem of response time of the sensor itself, which is present for all mass flow sensors, is particularly appreciable for the flow meter having a low flow, on the order of 10 to 50 cc/minute, for which the convergence time of the sensor can reach several minutes. These sensors are thus very difficult to calibrate precisely. The problem of response time, considered to be a critical point, has been approached in particular in the patent application FR 2 530 014, with a solution made by a special arrangement made of three coils wound around the tube end to end, with a coil upstream from the heating, an intermediate coil for measurement and supplemental heating, and a downstream coil for cooling between the temperatures created by the two first coils.
It is noted that in other systems, the current can be varied in a manner in order to maintain an always constant temperature, and from that the flow can be deduced. This thus involves the problem of control and thus a more complex electronic card (see EP 0 522 496 A1, Nippodenso, 1992).
The invention presented here proposes, to remedy some of the disadvantages cited, a new sensor having a very short response time.
According to a second goal of the invention, the sensor created in this manner allows low measurements.
According to a third goal of the invention, the process for manufacturing the sensor allows a manufacturing of sensors having precisely known characteristics which are reproducible and stable over time.
The present invention is a sensor for a mass flow meter comprising a capillary tube for the circulation of fluid in parallel with the main circuit of fluid circulation and designed to be integrated into a circuit for the circulation of gaseous fluids. The present for heating the capillary tube and mechanisms for measuring the temperature upstream and downstream from the heating mechanism and separate from the heating mechanism. The mechanisms for measurement of the temperature have the form of two resistors made by the deposition on the outside of the sensor tube, one of them upstream from the heating mechanism, the other downstream from this mechanism.
The separation of functions of heating and measurement makes it possible to choose materials better suited for each of the functions of heating and measurement, and thus for considerably improving the sensor performance.
The selection of resistors deposited on the tube leads to components not varying characteristics with time, contrary to the customary systems (wound resistors), hence, an increased reliability associated with the resistors having reproducible and stable characteristics over time.
According to particular devices, possibly combined, the sensor is characterized in that:
the two measurement resistors are placed symmetrically at a predetermined distance from the heating mechanism, corresponding to the zones presenting a maximum difference in temperature,
the capillary tube is made of stainless steel type 316L,
the capillary tube comprises an electrically insulating layer made of zirconia deposited at a thickness of several microns,
the measurement resistors are made of platinum,
the resistors have connection contacts at their ends having their own resistance which is many orders of magnitude lower than that of the resistors,
the heating mechanism is made in the form of a coil of wire made of an alloy of nickel and chromium in the proportions of 75% -20%.
These different arrangements correspond to a preferred embodiment of the sensor. The selection of resistors made of platinum makes it possible to obtain resistances which are extremely low and a value of resistance known with precision. Similarly, the selection of the alloy of the heating resistor makes it possible to obtain a very stable resistance over time.
The use of contacts at the ends of the resistors having their own resistance which is many orders of magnitude lower than that of the measurement resistors makes it possible, during the calculation of the variations in the resistance associated with the passage of fluid into the sensor, to consider the resistance of the contacts to be negligible.
The use of wire made of nickel chromium alloy for the heating mechanism makes it possible to have a resistance which is very slightly variable as a function of temperature, and thus a constant and known heating function.
The selection of an electrically insulating layer made by deposition at a thickness of several microns (and thus presenting a thermally negligible resistance in this layer by thin-film manufacturing), associated with the manufacturing of sensitive resistors also by deposition in thin-film manufacturing, leads to a response time of the sensor which is clearly shorter than the devices using a thick insulator (for example, an aluminum sleeve of existing devices) and wound resistors having a sizable thermal mass.
The invention is also intended for the process for manufacturing the sensor tube designed to be integrated into a mass flow meter, comprising the following steps:
calculation of the distance for positioning the resistors as a function of the equations characterizing the temperatures of the tube and the gaseous flow,
deposition of the insulating layer by electron gun, by adding oxygen into the enclosed space,
then deposition of resistors made of platinum, by electron gun, on the capillaries insulated by the zirconia layer, through the nickel masks, at a residual pressure lower than 10xe2x88x926 torr, at a thickness of several thousand angstroms.
then the deposition of the platinum contacts, by electron gun, on the capillaries insulated by the zirconia layer, through the nickel masks, at a residual pressure lower than 10xe2x88x926 torr, at a thickness of several microns.
The determination of the position of the points corresponding to a maximum temperature difference between the measurement resistors is done using the equations characterizing the temperatures of the tube, on the one hand, and of the flow, on the other hand, which the customary theoretical models do not allow with precision by a single equation. These calculations have been confirmed by numerical calculations made with simulation software.
The selection of deposition of insulating material and measurement components by electron gun makes it possible to obtain a contact of the resistors which is extremely close to the insulated tube, thus a response time of the sensor which is very short.
The previously known sensors, on the contrary, use a thick, adhered and non-deposited deposition, with questionable reliability of contact and thus questionable reproducibility of the measurement made by the sensors. It is also possible here to make a deposition layer reduced to 2.5 microns instead of tens of microns as currently known.
Finally, a reproducible procedure is created, with thus the capacity to produce exactly identical characteristics in a large series of sensors.
The selection of this technology makes it possible to also manufacture resistors on the curved surface of the sensors (cylindrical tube).
According to a preferred embodiment mode, the manufacturing process of the sensor tube is includes, at the end of processing, a step for annealing the tubes for one hour at 300xc2x0 C.
The description which follows, made in view of the attached drawings in the goal of explaining and not at all limiting, makes it possible to better understand the advantages, goals and characteristics of the invention.